NYC

At the southernmost end of Brooklyn, just off Dead Horse Bay, there’s a weather-beaten helipad where the New York Police Department keeps a gray unmarked twin-engine Bell 412 helicopter. Detective Brendan Galligan ushers me aboard. “We don’t really let people see this,” he says.

We climb in behind the pilot and find ourselves facing a console with three screens: One shows a map of the city; another, an interface for checking license plates and addresses; and the third, the view from a gyro-stabilized L-3 Wescam camera attached to the chopper’s nose. The camera can see clear across the city, in both the visible and the infrared slices of the spectrum; then it can broadcast the images to police headquarters using an onboard microwave transmitter.

The helicopter, part of New York City’s antiterror arsenal, takes off and climbs to 1,000 feet in the afternoon sunshine. Passing the Verrazano-Narrows Bridge, Galligan scans for suspicious trucks lingering on approach ramps. Over the Staten Island Ferry, he explains how police routinely use the chopper to look for boats that might be trailing too closely. Then, as we swing past the gaping World Trade Center site, the 22-year veteran adjusts the joystick to turn the camera eastward, filling the third screen with the towers of lower Manhattan: the center of the center of the bull’s-eye.

The New York Stock Exchange, the American Stock Exchange, the Federal Reserve Bank, City Hall, four major bridges and tunnels — a bomb at any of these places could kill hundreds, cost the city billions, and rattle the world financial system. Al Qaeda has hit lower Manhattan twice, in 1993 and 2001, and officials say that several other plots have been broken up since.

City agencies have done their best to harden the financial district in the years since 2001. Today, explosives-sniffing dogs and two truckloads of cops wearing military-style body armor and waving M-4 machine guns surround the flag-draped stock exchange. Black metallic barriers rise out of the asphalt, blocking traffic on Wall Street, while concrete planters and strategically parked trucks keep vehicles off Broad Street. Some of the other streets surrounding the exchange have been cut off to pedestrians, and only invited guests are allowed inside. “Closed since 9/11,” the guard tells visitors.

But you can’t block off every street or have a guard by every door. There’s no budget for that, and no one would want to live or work in that kind of armed camp anyway. “You can make a justification for putting bollards in front of every building,” says a former high-ranking NYPD counterterrorism official. “But pretty soon you can’t walk anywhere. People leave.”

So New York has an audacious blueprint to wrap a high tech cloak around lower Manhattan. It will provide the most sophisticated armor of any major urban area in the world — one that relies on brains as much as brawn, on barely visible technology as much as brute stopping power. And the chopper I’m in will be just a small piece of it.

Read moreNYC Is Getting a New High-Tech Defense Perimeter.